(1) Field of the Invention
The present invention relates to a load sensor which can detect a load at a high precision.
(2) Description of Related Art
As a technique detecting a load applied to each of mechanical parts constructing a machine, there has been known a pin type load sensor using a pin for coupling the parts used in a coupling portion of the machine as a load sensor, as shown in JP-A-10-38713. The pin type load sensor is structured such as to measure a strain of the pin generated by a load application by a strain measuring apparatus so as to detect the load. In this case, a metal resistance type strain gauge is used as the strain measuring apparatus.
However, in the case of the pin type load sensor in which the conventional strain gauge is used as the strain measuring apparatus, there is a problem that a zero point fluctuation of a strain gauge output is great at a time of being incorporated into a mechanical product. Further, since the conventional strain gauge is of a metal resistance type, an electric power consumption is great by being driven by a battery, so that there is a problem that the battery is consumed in a moment. Further, in the case of the load sensor utilizing the strain gauge, a wiring from the strain gauge to an amplifier tends to be affected by a noise, and there is a problem that an SN ratio is lowered.
Accordingly, in order to solve the problems, the inventors of the present invention have invented a pin type load sensor making use of a semiconductor strain sensor shown in JP-A-2005-114443, for achieving a pin type load sensor which has a small zero point fluctuation of an output, has a high noise resistance, and can be driven by a battery for a long time at a low electric power consumption. The semiconductor strain sensor uses an impurity diffusion resistance (hereinafter, refer to as a diffusion resistance) obtained by introducing an impurity to a single crystal silicon substrate as a strain sensitive resistance, and has a feature that a strain sensitivity is high and a zero point fluctuation is very small. Further, an output of the semiconductor strain sensor is such a great voltage that can detect the load without amplifying a signal by an amplifier or the like, and has a feature of being hard to be affected by the noise.
As shown in the prior art, in the case of the pin type load sensor using the strain gauge, since the strain sensitivity is low in the strain gauge, it is necessary to detect by amplifying the strain of the pin generated by the load application. Accordingly, a large hole is provided in an inner portion of the pin, the strain gauge is provided in an inner wall, and the strain generated in the inner wall is directly measure. In other words, the structure is made such that the strain at the strain measuring position in the pin inner portion is enlarged.
Further, there has been known a technique of putting a detection block into a hole of the pin so as to measure a strain of the detection block, however, in this case, the detection block has a complicated structure in such a manner that the strain of the pin can be amplified by the detection block. Further, the inner wall of the pin and the detection block directly comes into contact with each other, whereby the load applied to the pin is directly transmitted to the detection block, and the strain at the strain measuring position is enlarged.
However, in the case of the load sensor using the semiconductor strain sensor, since a main part of the semiconductor strain sensor is constituted by a single crystal silicon substrate, there is a risk that the strain sensor is broken if an excessive strain is generated in a pin type load cell at a time of applying a shock or the like. Accordingly, in the case that the semiconductor strain sensor is used in the pin type load cell, it is an important object to structure such that the excessive strain is not generated at the strain measuring position of the semiconductor strain sensor.